It is known to provide mains supply safety sockets wherein the live and neutral outlets are covered with an insulating plate which is adapted to retract when the ground pin of a three pin plug is inserted into the ground outlet of the socket. It is generally arranged in such systems for the ground pin to be slightly longer than both the neutral and live pins in order to facilitate the operation of the safety mechanism.
The major drawback with such a system is that the insulating plate is liable to jam. If it jams in the closed position the socket is unusable, and if it jams in the open position the socket is usable but the safety device is inoperative. Furthermore, such a system is not childproof in that it is readily appreciated that insertion of a ground pin, or any substitute therefor, will on its own retract the insulating plate, thereby overriding the safety mechanism. Indeed, this approach is sometimes adopted in order to connect the supply leads from an electrical appliance directly to the mains without first attaching a plug to the appliance, and is obviously highly unsatisfactory from a safety point of view.
U.S. Pat. No. 4,623,209 (Mangone) describes an electrical safety socket including normally open switches for connecting the live and neutral feeders to their respective socket outlets. The switches close when substantially parallel live and neutral pins of an electric plug are inserted into their corresponding socket outlets simultaneously. In such an arrangement, the insertion of each plug pin is adapted to pull a respective lever located external to the plug housing and articulated to a corresponding one of the switches. Such an arrangement allows independent operation of the switches and, since the levers are accessible from outside the plug housing, they may easily be operated independently of a plug being inserted into the socket. Consequently, foreign objects may be introduced into either socket outlet and the appropriate switch closed manually in order to complete the connection.
U.S. Pat. No. 4,008,403 (Rose) describes a safety socket for an electrical lamp, the shell and tip contacts of which are connected to respective supply feeders via normally open switches. The switches are operated by an electromagnetic relay which receives power via the shell contact when an external momentary make switch is closed. Additional safety is provided by splitting the shell contact into two halves such that electrical contact between them is effected only when a lamp is inserted into the socket. Thus, the split shell contact acts as switch for operating the electromagnetic relay.